Why Erenville succeeded as a character where others did not

[Iscah]

Heck this expansion we get told and shown what two different clans of the same race look like and how rare those children not only are born but survive just being born. So even if two races can be compatible it is highly unlikely that it would happen even if it was a socially acceptable to try and have more mixed races.

As Cleretic said, the high mortality is likely to be a secondary result of the hatchlings being mutants rather than just because they're mixed clan. The human-like races clearly can have mixed-race children with the normal number of heads.

There is still quite likely a practical aspect to avoiding having children with other races, though – different adaptations could clash in weird ways that are hard on the child, so it might be a deliberate decision to avoid that.